Why does climate change shrink in winter

Winter took a while in Europe to get going. But now it is on the move like it has not been for a long time. At the beginning of January the first cold wave swept across the continent, with double-digit minus temperatures and a lot of snow as far as Turkey. And now the next round is on: This week it will also be icy, from Tuesday on there should be double-digit minus temperatures in Germany again. Despite climate change?

No, especially when it comes to climate change, say some scientists. With the continuing warming of the earth, the weather is also mixed up. And possibly also the high-altitude wind called jet stream, which orbits the earth and usually drives rather mild Atlantic weather from the west to Europe, while it encircles the cold in the Arctic. When it gets really cold, this steady westerly wind is usually over: Then the jet stream throws wild arcs in which lows can shovel cold polar air and snow southwards. Such outbursts of polar air are what keep North America sinking in snow. And the most recent cold spell in Europe was also the result of a disrupted jet stream.

The phenomenon is not new, but climate change could increase its frequency. After all, there have been a striking number of cold winters in parts of Europe, North America and Asia since the 1990s. In Germany, too, the winters 2005/2006 and 2009/2010 were significantly colder than usual. Some researchers even interpret the data in such a way that it has recently become colder in the region in winter. "In Central Asia there is a clear trend towards colder winters," says Marlene Kretschmer from the Potsdam Institute for Climate Impact Research (PIK).

The mathematician is one of those who suspect one of the causes in the dramatic changes at the North Pole - of all places, where the warming is stronger than anywhere else on earth. The region has already heated up by several degrees Celsius, and the ice at the North Pole is shrinking at a dizzying pace. Last autumn, one negative record after the other was broken - in November the ice surface even shrank at times instead of growing. More than a million square kilometers are still missing from the normal January expansion, roughly the area of ​​Germany and France combined.

This has consequences, not just for the animal world. Missing ice exposes the dark ocean, so that it absorbs more heat in summer and can more easily release it in winter without the insulating layer of the ice. If that happens in the Barents Sea off Finland and Russia or in the Kara Sea further east, the thermal shock can disrupt the winds in the north and transport cold to Siberia and Europe. With such a phenomenon, Kretschmer's PIK colleague Vladimir Petoukhov was able to explain the icy winter 2005/2006 in a model (Journal of Geophysical Research, 2010). "I could imagine that the cold snap at the beginning of January would repeat the situation from then," says Petoukhov. "In any case, the Barents Sea is almost as ice-free as it was back then, we have to investigate that."

The heat surge from the open polar sea could have even more far-reaching consequences, up to the high layer of air called the stratosphere. There, many researchers believe, the disturbance can weaken the polar vortex: a characteristic band of wind that circles far higher than the jet stream over the North Pole, usually strong and steady in winter. A weakened polar vortex could in turn disrupt the jet stream and cause the polar cold to slosh south.

The ice-free Arctic acts like a pump that drives the cold south

Last year, Marlene Kretschmer and colleagues showed that the measurement data fit well with such a mechanism (Journal of Climate). A large blanket of snow in Russia could further intensify the effect. The ice-free, warm Arctic would sometimes be a kind of cold pump for Asia, North America and Europe. But is that really how it works? It has not been proven, even if much suggests it.

In the meantime, the research community is divided into those who suspect a strong influence of the ice surface and the skeptics who slow down - both camps will meet at a workshop in Washington DC at the beginning of February. Thomas Jung from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven will also be there. He sees himself more in the second group: it's not all that clear. "Certainly there are correlations," he says. "But it is difficult to deduce causes from this."

And even if they did, the question remains whether the impact of the shrinking ice cover would stand out in such a way. Models show that the atmosphere can create a lot of chaos on its own; Storms, mild periods, great cold. What of this has anything to do with the Arctic ice cover remains difficult to find out. Jung does not think the proposed mechanisms are strong enough to explain extremely cold winters such as 2005/06 and 2009/10, at least not in Germany: "The big question mark is Europe," he says. In Asia or North America, the influence of the Arctic is more evident, but Europe's main weather kitchen remains the North Atlantic, which complicates things.

Perhaps the current weather can soon bring more clarity. "This winter is very exciting for us," says Kretschmer: extremely little sea ice in the Arctic, already a lot of snow in Russia in autumn, perfect conditions for the Arctic cold pump. In fact, the polar vortex was very weak until the end of December. "So far everything seems to go very well together," says Kretschmer. Whatever the reason: First of all, it should stay cold.